In recent years, many aircraft manufacturers have sought improved ice protection systems to enable aircraft to safely fly in atmospheric icing conditions. Ice accumulations on the leading edge surfaces of various aircraft structures can seriously effect the aerodynamic characteristics of an aircraft. Examples of such aircraft structures include wings, engine inlets, and horizontal and vertical stabilizers. A leading edge is that portion of a surface of a structure that functions to meet and break an airstream impinging upon the surface of an aircraft structure. The impinging airstream is induced during flight. Conventional pneumatic de-icers, electrothermal de-icers and bleed air anti-icers have been used for many years to protect the leading edges of general aviation or commercial aircraft. These ice protection techniques are described in detail by Technical Report ADS-4, Engineering Summary of Airframe Icing Technical Data published by the Federal Aviation Agency, December 1963. In spite of these proven techniques, many aircraft manufacturers and operators have expressed a desire for new systems having better ice removal performance, longer life and decreased weight and energy requirements.
In response to this need, a class of systems has been developed that utilize skin deflection means to dynamically activate a thin deflectable outer skin upon which ice accumulates. The dynamic activation induces rapid motion in the thin deflectable skin sufficient to dynamically debond, shatter and expel an accumulated ice cap into surrounding airflow. As will be discussed more fully, the skin deflection means can take a variety of forms.
In some devices, the skin deflection means are combined with the thin deflectable outer skin to form a unitary de-icer. The unitary de-icer is generally formed in a thin sheet that can be subsequently bonded to the leading edge surface of an existing aircraft structure. The de-icer is usually designed to be removed from the aircraft structure and replaced in the field requiring the use of a replaceable adhesive such as 3M 1300L rubber cement. Examples are presented in U.S. Pat. No. 4,706,911 METHOD AND APPARATUS FOR DEICING A LEADING EDGE, Briscoe et. al. (hereinafter referred to as the Pneumatic Impulse Patent), U.S. Pat. No. 4,875,644 ELECTRO-REPULSIVE SEPARATION SYSTEM FOR DEICING, Adams et al. (hereinafter referred to as the Electro-Repulsive Patent), and U.S. Pat. No. 5,129,598 ATTACHABLE ELECTRO-IMPULSE DE-ICER, Adams et al. (hereinafter referred to as the Electro-Impulse Patent). In other devices, the skin deflection means are combined with the thin deflectable outer skin and a reinforcing structure thereby forming a unitary leading edge structure with integral de-icing capability. The de-icer is permanently bonded to the reinforcing structure necessitating replacement of the entire assembly upon failure of the de-icer. An example of this type of device is presented in U.S. Pat. No. 5,098,037 STRUCTURAL AIRFOIL HAVING INTEGRAL EXPULSIVE SYSTEM, Leffel et al. (hereinafter referred to as the Integral Expulsive System Patent). For the purposes of this application, the structure to which the de-icer is attached will be referred to as the "substructure." Examples of substructures include an existing aircraft structure having a leading edge surface and a reinforcing structure as discussed above.
As mentioned previously, the skin deflection means can take a variety of forms. In the Electro-Repulsive Patent, the skin deflection means comprises an upper array of conductors and a lower array of conductors. The upper conductors are substantially parallel to each other and to adjacent conductors in the lower layer. The upper conductors are connected in series with the lower conductors so that a single continuous conductor is formed that passes from the upper layer, around the lower layer, back around the upper layer, and so on. Upon application of an electrical potential to the input leads, current is developed in the upper conductors that is in the same direction in all upper conductors. Likewise, current is developed in the lower conductors that is in the same direction in all lower conductors, but opposite to the direction of the current in the upper conductors. As explained in the Electro-Repulsive Patent, maintaining a constant current direction in all the conductors of a layer greatly increases the separation force between the two layers.
After installation of the de-icer on a substructure, the upper and lower conductors are sandwiched between the structural member and a surface ply (the surface ply is analogous to a thin deflectable skin). Upon application of a high magnitude short duration current pulse, opposing electromagnetic fields in the upper and lower layers forcefully repel each other. This motion induces a dynamic motion into the surface ply which dynamically removes accumulated ice. As described in the Electro-Repulsive Patent, a current pulse that rises to between 2300 and 3100 amperes within 100 microseconds generates effective ice removal. A circuit for generating such a pulse is described in the Electro-Repulsive Patent. The circuit includes a pulse forming network, but this is not absolutely necessary.
Another form for the skin deflection means utilizing electromagnetic apparatus is illustrated by the Electro-Impulse Patent. A planar coil comprising at least one coiled conductor is sandwiched between a surface ply and a conductive substructure (such as the leading edge of an aluminum aircraft structure). Planar coils are described in great detail in U.S. Pat. No. 5,152,480 PLANAR COIL CONSTRUCTION, Adams et al. (hereinafter referred to as the Planar Coil Patent). As described in the Electro-Impulse Patent, a high magnitude short duration current pulse is applied to the coil. The current in the coil induces a strong rapidly changing electromagnetic field. The electromagnetic field generates eddy currents in the conductive substructure which, in turn, generates an opposing electromagnetic field. The two electromagnetic fields repel each other causing a repelling force between the coil and the substructure. The coil induces dynamic motion into the surface ply thereby dynamically removing accumulated ice. Effective ice removal is generated by a peak current of about 3000 amperes rising in a period of 100 microseconds. An electrical circuit for generating such a pulse is disclosed. The circuit is very similar to the circuit disclosed in the Electro-Repulsive Patent.
In the previous example, the skin deflection means is composed of a single unitary planar coil. A target may also be required if the substructure does not have sufficient electrical conductivity to effectively develop eddy currents. A target would be required with a fiber reinforced plastic substructure, or a conductive substructure that is too thin to effectively develop eddy currents. The target is a sheet of conductive material such as copper or aluminum that is located adjacent one surface of the coil. The coil and target are forcefully repelled from each other upon application of a high magnitude short duration current pulse to the coil due to opposing magnetic fields generated by current in the coil and by eddy currents in the target. This motion induces dynamic motion into the surface ply which dynamically removes accumulated ice. The target can be formed as a part of the substructure or can be formed as a part of the thin force and displacement generation means. Also, as described in the Electro-Impulse Patent, either the target or the coil can be located immediately subjacent the outer skin. The target applies the motive force to the skin if it is located subjacent the skin. Conversely, the coil applies the motive force to the skin if it is located subjacent the skin.
The Planar Coil Patent also teaches an electro-repulsive variation similar to the Electro-Repulsive Patent. Two mirror image unitary planar coils are superposed relative to each other and electrically connected so that upon application of a high magnitude short duration current pulse to each coil, current direction is opposite in each coil. Opposing electromagnetic fields are generated in the coils which causes each coil to forcefully repel the other. This motion induces a mechanical impulse into the surface ply which removes accumulated ice. This approach differs from the Electro-Repulsive Patent which utilizes a single conductor to form the upper and lower conductors.
A type of skin deflection means that utilizes pressurized gas is described in the Pneumatic Impulse Patent and the Integral Expulsive System Patent. A plurality of pneumatic impulse tubes extend in a spanwise direction subjacent a thin deflectable outer skin. The tubes and skin are supported by a fiber reinforced plastic substructure which together form a leading edge structure with integral de-icing capability. Special fittings are integrated into the tubes at various locations spaced along the span of each tube. A pneumatic impulse valve is attached to each fitting. A suitable valve is described in U.S. Pat. No. 4,878,647 PNEUMATIC IMPULSE VALVE AND SEPARATION SYSTEM, Putt et al. The valve contains a small volume (about 1 cubic inch) of high pressure air (500 to 5,000 psig). Upon activation by a solenoid, the valve quickly releases the pressurized air into each tube via the fitting. The expanding air pulse causes the tube to expand and induce mechanical motion into the skin thereby dynamically expelling accumulated ice. The expanding air pulse most preferably inflates the tube in less than 500 microseconds.
As evidenced by these patents, many variations of skin deflection means have been developed. The Electro-Repulsive Patent, Electro-Impulse Patent, Planar Coil Patent, Pneumatic Impulse Patent, and Integrated Pneumatic Impulse Patent provide examples of the types of structure that can serve as skin deflection means. In each example, the skin deflection means generates a force that causes the skin to be deflected away from the substructure. These patents are intended to be merely representative, and the types of structures that can serve as skin deflection means is not limited to the specific teachings of these patents.
Certain devices in the art are presented in FIGS. 1 and 2. The de-icers of FIGS. 1 and 2 have skin deflection means of the type that utilize compressed air as described by the Pneumatic Impulse Patent and Integral Expulsive System Patent. Unless noted otherwise, the following discussion applies equally as well to skin deflection means that utilize electromagnetic apparatus similar to those presented in the Electro-Repulsive Patent, Electro-Impulse Patent, and Planar Coil Patent. Referring to FIG. 1, a de-icer 100 is shown attached to a substructure 102 which serves to support the de-icer 100. An outer surface 122 meets and breaks an impinging airstream 119. Ice cap 115 is deposited by the airstream 119 during flight in atmospheric icing conditions. The section shown in FIG. 1 is a chordwise cross-section. The chordwise direction is defined as being approximately parallel to the direction of the impinging airstream 119 as it passes around the de-icer 100 and substructure 102. The de-icer 100 and substructure 102 also extend in a spanwise direction which is generally perpendicular to the chordwise direction. The de-icer and substructure can either be straight or have curvature in the spanwise direction. If de-icer 100 is applied to an engine inlet, the spanwise direction corresponds to the circumference of the inlet. In practicing the invention, the spanwise curvature can generally be ignored. Therefore, for the purposes of this application, the term "curvature" refers only to curvature measured in the plane of the chordwise section.
The outer surface 122 has a radius of curvature R that changes depending on the chordwise position along the outer surface 122. The radius of curvature R is measured perpendicular to the outer surface 122 in a chordwise plane. De-icer 100 and substructure 102 have an apex 120. The term "apex" is intended to refer to the portion of a de-icer and substructure underlying the area of the outer surface where the radius of curvature is smallest. The outer surface 122 defines a typical curvature wherein the smallest radius of curvature R is over the apex 120 and the radius of curvature R increases with distance from the apex 120.
De-icer 100 is comprised of a skin 104 and skin deflection means 103. Substructure 102 can be formed from metal, such as aluminum, or from fiber reinforced plastic, such as a plurality of reinforcing plies impregnated with plastic matrix (for example, plies of fabric formed from carbon, glass, or Kevlar.RTM. fibers impregnated with epoxy resin). The outer surface of the de-icer forms the outer surface 122.
The thin deflectable skin 104 is composed of an erosion resistant layer 105 and a backing layer 106. The erosion resistant layer can be formed from nearly any film having good erosion resistant properties. Titanium 15-3 alloy 0.005 inch thick and polyether-ether-ketone (PEEK) ranging from 0.007 to 0.016 inch thick have been used for erosion layer 105. The backing layer 106 can either support and reinforce the erosion layer, or it can serve to bond the erosion layer to the skin deflection means 103. Epoxy and nitrile phenolic film adhesives have been used for the backing layer 106.
The skin deflection means 103 has five expandable tubes 107.varies.111 that abut each other along an edge of each tube. Tubes 107-111 can be formed from plastic coated fabric, such as nitrile phenolic impregnated nylon fabric, or from rubber coated fabric such as neoprene coated nylon fabric. The tubes 107-111 are described in greater detail in the Pneumatic Impulse Patent and Integral Expulsive System Patent. One tube 109 overlies the apex 120. Tube 110 is shown inflated. Deflections of skin 104 over tubes 108 and 109 are shown by phantom lines 112 and 113 respectively. Tubes 107-111 are sequentially inflated by pulses of compressed air as described in the Pneumatic Impulse Patent or Integral Expulsive System Patent. Inflation of the tubes 107-111 induces dynamic motion in the skin 104 and ice cap 115 is debonded and shattered into side ice-pieces 116 and 118, and nose ice-piece 117, which are ejected into impinging airstream 119. Centerline 121 bisects the de-icer 100 and substructure 102. Depending on the angle of the incoming airflow in relation to the centerline 121, ice accumulation 115 could shift to predominantly one surface or the other. For example, if the incoming airflow rotates to below the centerline 121, the ice cap would shift back over tube 107 and forward over only part of tube 110. The amount of shift depends on the magnitude of the angle between the incoming airflow 119 and the centerline 121 which is a function of aircraft flight and airflow characteristics. Tubes 107 and 111 are provided to protect against shifts in the ice cap 115. They are normally activated sequentially with tubes 108-110 as part of a single activation cycle.
Referring now to FIG. 2, de-icer 200 represents another arrangement for the skin deflection means. De-icer 200 is shown attached to substructure 202 that has an apex 220. De-icer 200 comprises skin 204 and skin deflection means 203. The de-icer 200 and substructure 202 are bisected by a centerline 221, and de-icer 200 has an outer surface 222. The outer surface 222 has a radius of curvature R that changes with distance from the apex 220. Here, the skin deflection means 203 has only four tubes 207, 208, 210 and 211 arranged such that the edges of tubes 208 and 210 abut directly over the apex 220. Skin 204 includes a backing layer 206 and an erosion resistant layer 205. The substructure 202, skin deflection means 203 and skin 204 can be constructed from the same materials as the substructure 102, skin deflection means 103 and skin 104 of de-icer 100. An ice cap 215 is deposited by an impinging airstream 219 and is shown debonded and shattered into side ice-pieces 216 and 218, and nose ice piece 217. The ice cap 215 is debonded and shattered by activation of the skin deflection means 203 as discussed previously in relation to skin deflection means 103 of de-icer 100. As before, tubes 207 and 211 are provided to protect against shifts in the ice cap 215. De-icer 200 is shown in an activated state by inflation of tube 210. Deflected profile of skin 204 induced during subsequent inflation of tube 208 is shown as a phantom line 212. Tube 210 is inflated by a pulse of compressed air which forces the skin 204 to rapidly move outward. The motion of skin 204 during inflation of tube 210 causes the ice cap 215 over tube 210 to debond and shatter into ice-pieces 218 which are ejected into the airstream 219. During subsequent inflation of tube 208, ice cap 215 debonds and shatters over tube 208 and side ice-pieces 216 are ejected into the airstream 219. Nose ice-piece 217 is located over the area where tubes 208 and 210 abut. As shown, deflection of skin 204 over the edge of a tube is small in comparison to the deflection over the center of a tube. Therefore, activation of skin 204 over the apex 220 of de-icer 200 is much less than activation of skin 104 over the apex 120 of de-icer 100. De-icer 100 is generally more effective than de-icer 200 in removing ice over an apex. However, depending on the radius of curvature over the apex, neither may effectively remove ice.
Referring to de-icer 100 of FIG. 1 and de-icer 200 of FIG. 2, the radius of curvature R over apexes 120 and 220, respectively, can have an adverse effect on ice removal performance. De-icer 100A of FIG. 1A illustrates how the geometry of the leading edge can effect ice removal performance. De-icer 100A is shown attached to the substructure 102. Like numbered components of de-icer 100 of FIG. 1 and de-icer 100A of FIG. 1A are equivalent. A skin deflection means 103A is comprised of five tubes 107A-111A. Tubes 107-111 are identical to tubes 107A-111A except for the width of each tube. The tube 109A overlying the apex 120 is wider than tube 109. Tube 109A is shown inflated. Due to the position dependent curvature of the outer surface 122, tube 109A tends to inflate on the sides, where the radius of curvature is greater, away from the apex 120, where the radius of curvature is lesser. Tube 109A has an outer wall 109A' which is pulled down over the very tip of the apex 120 resulting in almost no force application to the skin 104 over the tip of the apex 120. Therefore, side ice-pieces 116 and 118 are removed, but nose ice-piece 117 located over the apex 120 is not removed. This phenomenon has been observed in numerous icing wind tunnel tests.
Referring to de-icer 100 of FIG. 1, tube 109 also has an outer tube wall 109'. If the curvature of the apex 120 is not too great, and the width of tube 109 is narrow enough, the outer tube wall 109' can deflect outward, as shown in FIG. 1, and apply force to the skin 104 over the tip of the apex 120 resulting in skin deflection 113. Ice piece 117 will be ejected. De-icer 200 of FIG. 2 may provide a viable solution depending on geometry. As a general guideline, de-icer 100 of FIG. 1 is suitable if the radius of curvature R over apex 120 is greater than about 1.0 inch. In contrast, de-icer 200 of FIG. 2 is suitable if the radius of curvature of apex 220 is between about 0.5 and 1.0 inch. However, de-icer 200 can be unsatisfactory for use with leading edge geometries having a radius of curvature R over apex 220 less than about 0.5 inch. A propeller blade represents a type of leading edge geometry that often has an apex radius of curvature of less that 0.5 inch. An effective means of removing ice over an apex having a small radius of curvature is desired.
In addition to ice removal performance, life of a de-icer represents another very important consideration. For the purposes of this application, de-icer life is defined as the length of time a de-icer can continuously operate before the de-icer mechanically fails. The components of the de-icer are subjected to a stress cycle each time the de-icer is activated. These stress cycles accumulate and eventually cause a de-icer to mechanically fail due to fatigue. There are two ways to increase the life of a de-icer similar to de-icers 100 or 200 without changing materials. The cycle rate can be decreased (fewer cycles per minute), or the stress levels can be reduced. Reducing the cycle rate usually is not an option because flight conditions and ice accumulation characteristics of an aircraft are usually fixed.
The other option, reducing the stress levels, is limited because skin deflection in de-icers similar to de-icers 100 or 200 is achieved predominantly by stretching the skin. Referring to de-icer 100 of FIG. 1, the substructure 102 is relatively rigid and stretching the skin 104 is the only way deflection of skin 104 over any of tubes 107-111 is achieved. Referring to de-icer 200 of FIG. 2, this is also true of skin 204, tubes 207-211 and substructure 202. Referring to de-icer 100 of FIG. 1, a maximum deflection 114 in skin 104 over tube 110 is presented. In general, the maximum deflection 114 ranges between about 0.020 inch to about 0.060 inch. The maximum deflection 114 depends mostly on two variables; (1) modulus of elasticity of the skin 104, and (2) the magnitude of force generated by tube 110. For a given set of materials, the maximum deflection 114 can be achieved only by increasing the force generated by tube 114 to a sufficient magnitude. Increasing the force increases stresses in tube 110 and skin 104 thereby decreasing life. Referring to de-icer 200 of FIG. 2, the same is also true in relation to maximum deflection 214 of skin 204 over tube 210 of de-icer 200. Therefore, a means of obtaining a maximum deflection in a thin deflectable skin while maintaining lower stresses in the skin and skin deflection means is desired. Decreasing the stresses in the skin and skin deflection means results in a de-icer having longer life.
In addition to ice removal, energy consumption and weight are also of primary importance. De-icers similar to de-icers 100 and 200 have an "active area." The term "active area" refers to that portion of the outer skin that is dynamically activated by the thin force and displacement generation means in a manner that removes ice accumulations. For example, the active area of deicer 100 includes any area of skin 104 covering tubes 107-111 and the active area of de-icer 200 includes any area of skin 204 covering tubes 207-211. Normally, as evidenced by de-icers 100 and 200, the surface area of the skin deflection means 103 and 203 must equal the active area. Reducing the surface area of the skin deflection means reduces the energy consumption of the de-icer. However, the active area required for a particular application is usually fixed. Therefore, a way of reducing the surface area of the skin deflection means without reducing the active area is desired in order to provide a de-icer having decreased energy consumption. Also, since the skin deflection means represent a significant portion of the weight of a dynamic de-icer, reducing the surface area of the skin deflection means in relation to the active area should also reduce weight.
In the context of a pneumatic impulse embodiment, the force required to deflect the outer skin bears on energy consumption and weight in a manner that is even less apparent. For de-icers similar to de-icers 100 and 200 of FIGS. 1 and 2, the pulse propagation distance generally decreases as the modulus of the skin increases. Pulse propagation distance refers to the distance from a valve along the span of a tube over which ice is effectively removed. In general, skin deflection and dynamics decrease with distance from a valve because the pulse of compressed air is constantly expanding and the peak pressure inside the tube decreases with distance from the valve. For example, if the erosion layer 105 or 205 is formed from 0.005 inch thick 15-3 titanium alloy, the pulse may generate effective ice removal about two feet on either side of a valve. Therefore, the distance between valves would be about four feet in order to provide effective ice removal along the span of a tube. Four valves per tube would be required for a sixteen foot span. For an ice protector having five tubes, a total of twenty valves would be required. By increasing the pulse propagation distance, the space-between valves can be increased. Increasing the space between valves reduces the number of valves and the total weight of the system. Reducing the number of valves also increases the reliability of the system by reducing the number of mechanical components. Therefore, means of increasing the pulse propagation distance between valves is desired in order to increase reliability and decrease energy consumption and weight.
The devices described above represent advancements over previous de-icing systems. In spite of these advancements, means of improving ice removal performance, life, reliability, weight, and energy consumption are of continuing interest. In particular, a de-icer is desired exhibiting the excellent ice removal performance typical of the devices described above while having increased life, reduced weight, and reduced energy consumption.